For the production of a synthetic quartz glass, there is well known a process of producing it by densifying a silica porous body mainly obtained by flame hydrolysis of a silicon halide at high temperatures. Since the thus obtained synthetic quartz glass body has passed through the flame hydrolysis step, it contains a large amount of an OH group in the silica porous body. If a large amount of an OH group is present in the synthetic quartz glass, the viscosity of the synthetic quartz glass is lowered, whereby the heat resistance is lowered. Accordingly, the subject synthetic quartz glass causes deformation and is not preferable for applications of quartz glass jigs for the semiconductor industry to be used at 1,000° C. or higher.
As a countermeasure thereto, JP-A-54-127914 discloses the preparation of an anhydrous glass matrix by thermally oxidizing or hydrolyzing a glass forming raw material to deposit a glass forming substance containing silicon dioxide (SiO2) as the major component on the end faces of a supporting rod, thereby preparing a porous glass and exposing this porous glass sintered body to a glass forming raw material gas containing a halogen element at from 800 to 1,000° C., followed by transparent vitrification. As the glass forming raw material gas, SiCl4, SiBr4, GeCl4, BBr3, POCl3, PCl3, and the like are exemplified.
Since the synthetic quartz glass obtained by the foregoing process contains less impurity, it has been expected as a substitute for a quartz glass raw material made of a natural quartz crystal as the raw material in the semiconductor manufacturing steps, but deformation in the high-temperature manufacturing steps has been recognized as a remarkable drawback.
Even if the OH group in the quartz glass is completely dehydrated by a dehydration treatment using the foregoing chlorine based gas, there was a limit in the value of increasing viscosity. Though this value was slightly larger than that of an undehydrated synthetic quartz glass, it was explicitly smaller than that of a natural quartz glass. In general, the viscosity of the synthetic quartz glass (poise being used as a unit of the viscosity in the present invention) is 11.4 for the undehydrated article, 11.6 for the dehydrated article, and 11.9 or more for the natural quartz glass, respectively in terms of a value of log η at 1,280° C.
As a process of producing a highly heat resistant glass, JP-A-3-83833 discloses a method of containing Al. However, according to this method, there was involved a problem that impurities such as metal impurities are contained.
Also, as a gas for removing the OH group in the porous body, ammonia or an ammonia based gas can be used in place of the halogen based gas such as chlorine. However, in this case, bubbles are frequently generated in the glass after transparent vitrification due to an NH group remaining after the dehydration reaction, thereby allowing no commercialization to be realized.
As a method of solving the problem of the generation of bubbles, JP-A-7-300324 proposes a method in which a nitrogen-doped porous body obtained by a heating treatment in an ammonia-containing atmosphere is baked by heating in a non-oxidative atmosphere, and the subject baked body is subjected to a high-temperature heating treatment under a high pressure of 500 kg/cm2 or more in the temperature range of from 1,400° C. to 2,000° C. However, according to the subject method, since the heating treatment under a high pressure is required, a special device was necessary.
Also, in the case where the heating treatment temperature exceeds 1,000° C., the quartz glass jig irregularly reflects infrared rays, and the temperature in a furnace becomes non-uniform. There were thus caused problems such as the matter that the material quality of a silicon wafer becomes instable.
On the other hand, as a process of producing a black quartz glass, there has hitherto been known a method of coloring it black by adding a metal oxide such as copper oxide, manganese dioxide, chromium sesquioxide, iron sesquioxide, and vanadium pentoxide (JP-A-4-254433, etc.). However, according to the method of using a blackening agent, it was difficult to obtain a completely uniform black glass.
In the meantime, in the semiconductor manufacturing steps, it is thought that an alkali metal such as Na, K, and Li is an element to bring about a defect on a device. In the case where the foregoing metallic contamination is brought from a treatment furnace in the heating treatment step of a silicon wafer, a quartz tube and a quartz boat for setting the silicon wafer play a role to prevent the contamination. However, in electrically fusible low-OH natural quartz or chlorine-free synthetic quartz-made quartz glass raw materials, the foregoing effect for preventing the foregoing contamination was low.